The difficulties encountered in bonding rigid components that have different coefficients of thermal expansion (CTEs) are well recognized. Where components are joined in a thermal operation, such as fusion, the components may separate, or fracture, on cooling, particularly if the CTE differential is over 20.times.10.sup.-7 /.degree. C. Even where a cold seal can suffice, there remains the tendency for failure if the article is subjected to thermal changes in service. The situation is further complicated where one of the components is glass, since a glass surface tends to check whenever the glass is stressed in tension above a critical level. For this reason, laminates involving glass are much more sensitive than other materials to equal expansion differences.
Accordingly, special care has been exercised, and special materials developed, to produce composite articles, such as glass-metal or glass-polymer combinations. The use of an elastomeric adhesive is widely practiced, but few of these materials meet the stringent requirements for laminates involving glass where long term optical clarity and long term adhesion retention are both equally important.
Silicones are elastomeric adhesives which are well known to show excellent adhesion to glass. They can also exhibit excellent optical clarity. Adhesion to other surfaces may be less than satisfactory, however. For example, after thermal cycling between 80.degree. C. and -40.degree. C. for an extended period, a silicone bond between a rhodium plated silver ornament and glass surfaces failed at less than 20 lbs. pull due to lack of adhesion to the metal surface. In contrast, a good seal will withstand a pull of over 100 lbs. Consequently, adhesives other than silicones must be employed for certain laminate applications.
For such purposes, epoxies have often been considered. However, they inherently tend to be rigid and brittle, rather than elastic and flexible. Many formulation adjustments are known which increase flexibility; however, when these were evaluated, it was found that they generally resulted in the loss of other desirable properties. It has been particularly difficult to formulate low modulus, epoxy adhesives which exhibit high elongation without sacrificing the desired high stoichiometric ratio between the epoxy and the curing agent. An epoxy tended to transition from a strong, tough material to a soft, weak material whenever the modulus was lowered by means of traditional formulation changes when the stoichiometric ratio was maintained over about 75%.
Recently, numerous applications have arisen where it would be very desirable to join a glass component with a rigid plastic or metal body having a very different CTE value. Most glasses of commercial or ornamental interest have CTE values below 100.times.10.sup.-7 .degree. C. In contrast, metals, such as iron alloys and precious metals, and rigid plastics, such as epoxies, tend to have CTEs much above 100.times.10.sup.-7/ .degree. C.
One such application occurs in connection with certain works of art. A casting of precious metal is bonded to a molded body of an art glass having a high lead content. Precious metals, such as silver and gold, have CTEs on the order of 150 to 200.times.10.sup.-7 /.degree. C., whereas of the lead glass is on the order of 90.times.10.sup.-7 /.degree. C. Such a difference precludes bonding, either directly, or by a vitreous seal.
It is possible to join the bodies by thermally softening the glass, but the metal tends,to separate on cooling, usually taking a fragment of glass with it. For this application, then, a bonding material must be tough, and must have good tensile strength and elongation, while having a low enough modulus to prevent excessive stresses at the glass interface. The tensile strength should be at least 250 psi, preferably over 500 psi; the modulus should be less than 10.sup.4 psi; and the elongation should be at least 50%.
Another application is an ophthalmic, glass-plastic, laminated lens. Such a composite body is described in U.S. Pat. No. 4,793,703 (Fretz, Jr.); also in U.S. Pat. No. 5,064,712 (Fretz, Jr.). Both patents describe a three-layer composite lens composed of an inorganic glass layer, a rigid organic plastic layer, and a flexible organic adhesive. The flexible adhesive is the product of an epoxy mixture that is cured at room temperature with an aliphatic amine curing agent.
This lens application, both because of the larger bonding area involved and the higher coefficient of thermal expansion of the plastic lens, requires an adhesive exhibiting an even lower modulus, with a moderate tensile strength and a high elongation value. Thus, the 10% secant modulus must not exceed 500 psi and is preferably less than 200 psi; the tensile strength must be over 100 psi, and preferably over 200 psi; the elongation at least 75%, and preferably over 100%.
In any product application, retention of properties on aging is critical. Both adhesion and cohesion must be retained upon aging to maintain adequate performance. Thus, loss of elongation or tensile strength may lead to delayed failure. An increase in modulus would, of course, be undesirable, since the adhesive would become less able to compensate for any thermal expansion difference that occurs. Moreover, in optical applications, long term retention of optical clarity and freedom from color are critical.